This invention relates to an optical pickup device used for recording information signals on an optical disc or reproducing information signals recorded on the optical disc, and an optical disc recording and/or reproducing apparatus employing this optical pickup device. More particularly, it relates to the optical pickup device and the recording and/or reproducing apparatus for selectively recording and/or reproducing information signals for first and second optical discs of different recording densities using two light beams having different wavelengths.
The optical discs may be classified into a first optical disc of a standard recording density and a second optical disc that is able to record the information signals to a higher recording density than the first optical disc. In the second optical disc, the interval between neighboring pits formed on a substrate is set so as to be narrower than that in the first optical disc, the pit size is reduced, or the track pitch of the recording tracks constituted by pit strings is reduced, for achieving high recording density. In the second optical disc, having the high recording density, the spot diameter of a light beam on a signal recording surface for recording or reproducing information signals needs to be smaller than that of the light beam on the signal recording surface of the first optical disc. To this end, the optical pickup device used for recording or reproducing information signals for the second optical disc uses a light source radiating a light beam of a wavelength shorter than that of the light beam used for the first optical disc and an objective lens having a numerical aperture NA larger than that of an objective lens used for the first optical disc.
On the other hand, since the optical pickup device for recording information signals on the second optical disc or reproducing information signals recorded on the second optical disc employs an objective lens with a larger numerical aperture, the substrate of the second optical disc needs to be thinner in thickness than that of the first optical disc in order to assure tilting tolerance of the optical disc loaded on a disc rotating mechanism and which is run in this state in rotation. If, in such a case, an optical pickup device for the second optical disc is used to record/reproduce the information signals or the first optical disc, spherical aberration is produced because of the substrate of the first optical disc being thicker in thickness than the substrate of the second optical disc to render it impossible to record or reproduce correct information signals.
Meanwhile, in a disc recording and/or reproducing apparatus, employing an optical disc as a recording medium, it is desired that first and second optical discs having different recording densities be recorded and/or reproduced on a common deck. For reproducing the first and second optical discs on the common recording and/or reproducing apparatus, it is contemplated to have an optical pickup for the first optical disc and an optical pickup for the second optical disc built into the apparatus, to discriminate the optical disc type by a discriminating unit, adapted for discriminating the disc type, and to switch between the optical pickup devices based on the result of discrimination.
There has also been proposed an apparatus adapted for recording or reproducing information signals for the first and second optical discs of different recording densities using an optical pickup employing a common light source and a common objective lens. The optical pickup device, used in this apparatus, includes a light source for radiating a light beam used for the second optical disc, an objective lens of a larger numerical aperture, used for the second optical disc, and a light stop mechanism for mechanically limiting the aperture of the objective lens when recording and/or reproducing the first optical disc. When recording or reproducing information signals for the first optical disc, the optical pickup device limits the objective lens aperture by the light stop mechanism to give a spot diameter suited to the recording density of the first optical disc, corrects the spherical aberration caused by the first optical disc being thinner in substrate thickness than the second optical disc, and illuminates a light beam on a signal recording surface of the first optical disc. When recording or reproducing the information signals for the second optical disc, the optical pickup device illuminates a light beam on the signal recording surface of the second optical disc without limiting the objective lens aperture by the light stop mechanism.
Meanwhile, if both the optical pickup device for the first optical disc and the optical pickup device for the second optical disc are built into a sole disc recording and/or reproducing apparatus, the apparatus itself is bulky in size to render it difficult to reduce the apparatus size.
Also, in the optical pickup device in which, for recording or reproducing information signals by a common optical pickup device for the first and second optical discs having different substrate thicknesses and different recording densities, there is provided a mechanical stop mechanism for limiting the objective lens aperture, the structure of the optical pickup device is complicated to render it difficult to reduce the device size.
It is an object of the present invention to provide a novel optical pickup device and a novel optical disc recording and/or reproducing apparatus which can be used in common for optical discs with different recording densities.
It is another object of the present invention to provide a novel optical pickup device and a novel optical disc recording and/or reproducing apparatus, which can be used in common for different optical discs of different recording densities, can be simplified in structure and reduced further in size.
It is still another object of the present invention to provide a novel optical pickup device and a novel optical disc recording and/or reproducing apparatus which allows for correct recording or reproduction of information signals for any optical discs having different recording densities.
It is yet another object of the present invention to provide a novel optical pickup device and a novel optical disc recording and/or reproducing apparatus which, even if optional optical discs with different recording densities are used, correct focussing error signals and tracking error signals can be produced, and which allows a light beam to scan a recording track of an optional optical disc to assure correct recording or reproduction of information signals.
For accomplishing the above objects, the present invention provides an optical pickup device including a first light source for radiating a first light beam, a second light source for radiating a second light beam having a wavelength different from that of the first light beam, an objective lens for converging the light beam radiated by the first light source or the light beam radiated by the second light source, a beam splitter having substantially the same transmittance to the light beam radiated by the first light source and to the light beam radiated by the second light source and adapted for separating the light beam radiated by the first light source or the light beam radiated by the second light source from the first light beam or the second light beam incident via the objective lens, and a hologram element arranged between the objective lens and the beam splitter for limiting the aperture of the first light beam or the second light beam and for correcting the spherical aberration.
The beam splitter has transmittances for the P polarization component and the S polarization component of the first light beam and the second light beam of 70% to 80% and 0% to 10%, respectively. Also, in the wavelengths of the first light beam, the phase difference between the P and S polarization components and the second light beam is larger than 10xc2x0.
The optical pickup device according to the present invention also includes a phase compensation element for compensating the phase difference between the P polarization component and the S polarization component of the first light beam or the second light beam, reflected by an optical disc to fall on the beam splitter via an objective lens so as to be separated by the beam splitter, and a photodetector for receiving the first light beam or the second light beam transmitted through the phase compensation element.
The photodetector receiving the first or second light beam, compensated for phase difference between the P polarization component and the S polarization component by the phase compensation element, is able to detect the light beam accurately.
The optical pickup device according to the present invention also includes a diffraction grating for generating at least three diffracted light beams from the first light beam radiated from the first or the light beam radiated from the second light source, and detects tracking errors by the three-beam method.
The optical pickup device according to the present invention includes a further beam splitter arranged on an optical path between the first and second light sources and the beam splitter for reflecting at least one of the first light beam radiated by the first light source and the second light beam radiated by the second light source and for transmitting the other light beam.
An optical disc recording/reproducing apparatus according to the present invention includes the above defined optical pickup device, a focussing error signal generating unit for generating focussing error signals based on an output signal from a photodetector provided on the optical pickup device and/or a tracking error signal generating unit for generating tracking error signals based on an output signal from a photodetector provided on the optical pickup device. The objective lens position is controlled based on the focussing error signals and/or the tracking error signals outputted by the focussing error signal generating unit and the tracking error signal generating unit, while the light beam focussing position and the illuminating position of the light beam on the signal recording surface of the first or second optical disc is controlled by this objective lens to realize accurate scanning of the recording tracks by the light beam.
Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments of the invention.